The present invention relates to the field of solid state lighting, and in particular to a LED string constituted of a plurality of serially connected LED strings, each provided with a controlled bypass path.
Light emitting diodes (LEDs) and in particular high intensity and medium intensity LED strings are rapidly coming into wide use for lighting applications. LEDs with an overall high luminance are useful in a number of applications including backlighting for liquid crystal display (LCD) based monitors and televisions, collectively hereinafter referred to as a matrix display, as well as for general lighting applications.
In a large LCD matrix display, and in large solid state lighting applications, such as street lighting, typically the LEDs are supplied in a plurality of strings of serially connected LEDs, at least in part so that in the event of failure of one string at least some light is still output. The constituent LEDs of each LED string thus share a common current.
In order to supply a white backlight for the matrix display one of two basic techniques are commonly used. In a first technique strings of “white” LEDs are utilized, the white LEDs typically comprising a blue LED with a phosphor which absorbs the blue light emitted by the LED and emits a white light. In a second technique individual strings of colored LEDs are placed in proximity so that in combination their light is seen a white light. Often, two strings of green LEDs are utilized to balance each single red and blue LED string.
In either of the two techniques, the strings of LEDs are in one embodiment located at one end or one side of the matrix display, the light being diffused to appear behind the LCD by a diffuser. In another embodiment the LEDs are located directly behind the LCD, the light being diffused by a diffuser so as to avoid hot spots. In the case of colored LEDs, a further mixer is required, which may be part of the diffuser, to ensure that the light of the colored LEDs is not viewed separately, but rather mixed to give a white light. The white point of the light is an important factor to control, and much effort in design in manufacturing is centered on the need to maintain a correct white point in the event that colored LEDs are utilized.
LEDs providing high luminance exhibit a range of forward voltage drops, denoted Vf, and their luminance is primarily a function of current. For example, one manufacturer of LEDs suitable for use with a portable computer, such as a notebook computer, indicates that Vf for a particular high luminance white LED ranges from 2.95 volts to 3.65 volts at 20 mA and an LED junction temperature of 25° C., thus exhibiting a variance in Vf of greater than ±10%. Furthermore, the luminance of the LEDs vary as a function of junction temperature and age, typically exhibiting a reduced luminance as a function of current with increasing temperature and increasing age. In order to provide backlight illumination for a portable computer with an LCD matrix display of at least 25 cm measured diagonally, at least 20, and typically in excess of 40, LEDs are required. In order to provide street lighting, in certain applications over 100 LEDs are required.
In order to provide a balanced overall luminance, it is important to control the current of the various LED strings to be approximately equal. In one embodiment, as described in U.S. patent application Ser. No. 11/676,313 to Korcharz et al, entitled “Voltage Controlled Backlight Driver”, filed Feb. 19, 2007 and published as US 2007/0195025 Aug. 23, 2007, the entire contents of which is incorporated herein by reference, this is accomplished by a controlled dissipative element placed in series with each of the LED strings. In another embodiment, binning is required, in which LEDs are sorted, or binned, based on their electrical and optical characteristics. Thus, in order to operate a plurality of like colored LED strings from a single power source, at a common current, either binning of the LEDs to be within a predetermined range of Vf is required, or a dissipative element must be supplied to drop the voltage difference between the strings caused by the differing Vf values so as to produce an equal current through each of the LED strings. Either of these solutions adds to cost and/or wasted energy. In order to utilize a plurality of colored LED strings mixed to provide a white light a color manager is further required, which yet further adds to cost.
Portable computers typically exhibit a large range of available input voltages. For example, when operating from battery power, the portable computer must be operative when the battery output has declined to approximately 5.5 volts. When connected to an AC mains via a power adapter, the portable computer must be operative for voltages well in excess of the lowest battery voltage, typically up to 28V DC. Thus, any solution must be operative over a wide input voltage range.
Prior art portable computers with an LCD matrix display of greater than 25 cm diagonally measured exhibit a plurality of short LED strings. Each LED string requires a maximum voltage of typically no more than 60 volts DC. Such a voltage is easily generated from the wide ranging DC input source, however to achieve a substantially uniform backlight one of the binning and the dissipative solution described above is required.